A polyfunctional cyanate ester compound is polymerized or cured by heating. West German Pat. No. 1,190,184 discloses a process for polymerizing a polyfunctional cyanate ester compound in the presence of a Lewis acid, phosphoric acid, hydrochloric acid, sodium hydroxide, tributyl phosphine, trimethyl amine, phospholin-.DELTA..sup.3 -1 oxo-1-phenyl or the like. In addition, U.S. Pat. No. 3,694,410, discloses carrying out of the polymerization in the presence of a metal chelate, such as an acetyl acetone metal salt.
It takes a long time to carry out the polymerization in the absence of any catalyst. The cured products obtained by the process of West German Pat. No. 1,190,184 have poor electrical properties. In addition, in U.S. Pat. No. 3,694,410, solubility of the metal chelate to the resin composition is poor, so a special solvent is necessary in order to enhance the solubility.